1. Field of the Invention
The present invention relates to compositions and methods employing this composition for controlling and/or inhibiting the formation, deposition and adherence of calcium deposits on surfaces of an aqueous system employed in the process of extracting precious metals.
2. Brief Description of the Background Art
Most industrial waters contain alkaline earth metal cations such as, for example, calcium, barium, magnesium, etc., and several anions such as bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, fluoride, etc. When these anions or cations or combinations thereof are present in concentrations which exceed the solubility of their reaction products, precipitates form until these product solubility limitations are no longer exceeded. For example, when calcium ion and sulfate ion or calcium ion and carbonate ion concentrations exceed the solubility of their calcium sulfate or calcium carbonate reaction products, respectively, a solid phase of calcium sulfate or calcium carbonate will form.
Solubility concentrations are exceeded for various reasons, such as partial evaporation of the water phase, a change in pH, pressure or temperature, or the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on surfaces of an aqueous system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow and facilitates corrosive processes and biological activity. Scaling is an expensive problem in many industrial systems, causing delays and shutdowns for cleaning and removal.
Calcium sulfate scaling is particularly troublesome in the mining of refractory grade ores using oxidation methods such as, for example, but not limited to, flash chlorination, roasting with air or pressure oxidation wherein aqueous ore slurries are oxidated under high temperatures generally greater than 400.degree. Fahrenheit and/or pressure conditions generally greater than 400 pounds per square inch to prepare the ore for leaching. It will be appreciated that calcium sulfate scaling occurs in autoclaves, flash chlorination vessels, roaster apparatus, and in leaching facilities such as, for example, in bioleaching operations, and on clarifier rakes, pumps, and related piping employed in the process of extracting precious metals.
U.S. Pat. No. 3,928,196 discloses the use of copolymers of 2-acrylamido-2-methylpropylsulfonic acid and acrylic acid as scale inhibitors.
U.S. Pat. No. 4,640,793 discloses the use of admixtures containing carboxylic acid/sulfonic acid polymers and phosphonates or polycarboxylates as scale and corrosion inhibitors.
U.S. Pat. No. 4,618,448 discloses the use of polymers comprising an unsaturated carboxylic acid, an unsaturated sulfonic acid and an unsaturated polyalkylene oxide as scale inhibitors.
U.S. Pat. No. 4,933,090 discloses the use of hexamethylenediamine tetramethylenephosphonic acid and diethylene pentamethylenephosphonic acid in combination with carboxylic/sulfonic acid polymers to control silica/silicate deposition.
Compositions containing hexamethylenediamine tetramethylenephosphonic acid, polyacrylic acid and a carboxylic/sulfonic acid polymer are used in papermaking to control calcium sulfate scaling.
European Publication No. 302651B1 discloses the use of carboxylic acid/sulfonic acid copolymers in combination with organic phosphonates and acrylic acid-type polymers as calcium carbonate scale inhibitors in highly alkaline aqueous systems.
None of these, however, disclose or suggest the instant invention, which relates to calcium and/or sulfate control on the process of extracting precious metals.
It will be understood by those skilled in the art that the process of extracting precious metals includes, but is not limited to, for example, controlling and/or inhibiting calcium sulfate compounds and/or calcium carbonate compounds on metallic, activated carbon and the surfaces of one or more aqueous systems involved in heap and vat leaching, carbon-in-pulp leaching, carbon-in-leach leaching, activated carbon leaching and adsorption recovery systems, or mill water circuits used to carry out the cyanidation process for extracting precious metals from crude ores. It is well known by those skilled in the art that the cyanidation process is combined with the use of activated carbon to recover precious metals such as, for example, gold or silver from large volumes of solutions or slurries containing water soluble cyanide salts of the precious metals created by the leaching step of the cyanidation process. Because oxidizing conditions must be maintained in order for the cyanidation process to proceed, about 1 to 2 pounds per short ton of ore of sodium hydroxide or lime is added to keep the system at an alkaline pH of about 10-11. Acid is generated during cyanidation and the alkaline pH prevents cyanide degeneration, which can lead to the formation of deadly HCN gas. While lime is significantly less expensive than sodium hydroxide in achieving alkaline pH's, it suffers from the disadvantage of causing the formation of calcium carbonate scale deposits at various points in the aqueous systems involved in the cyanidation process.
Because of the high pH's and alkalinity involved in the cyanidation processes described above, conventional agents such as, for example, polyacrylates and polymaleic anhydrides used to control calcium carbonate scale in more traditional areas such as boilers or cooling water do not give satisfactory performance in the process of extracting precious metals in an aqueous system.
Thus, it is a significant contribution of the composition and method of the present invention that by the addition of effective amounts of the present composition it is possible to substantially inhibit the formation of such calcium carbonate scale deposits, thereby allowing the use of the less expensive lime, rather than sodium hydroxide, in maintaining alkaline pH's for the cyanidation process.
Further, for example, in the severe environment of the metal mining circuits described hereinabove, conventional scale inhibitors such as polymaleates and polyacrylic acid have generally proven to be ineffective as calcium sulfate scale inhibitors. By contrast, the compositions of the instant invention, as described herein, effectively control calcium sulfate scaling in these mining circuits employing water.
Thus, in spite of this background material, there remains a very real and substantial need for a composition and methods employing the composition for substantially controlling and/or inhibiting the formation, deposition and adherence of calcium carbonate and/or calcium sulfate scale deposits on surfaces of an aqueous system employed in the process of extracting precious metals.